This invention relates to food trays and other thermoformed structures comprising a core layer with outer layers of polyethylene terephthalate reinforced by glass flakes.
Glass flakes have been used to reinforce polyethylene terephthalate (PET). U.S. Pat. No. 4,555,439, to Ueeda et al., discloses rolled sheet-like materials having superior physical properties, in particular toughness and heat resistant properties. These materials comprise 20-90% thermoplastic resin and 10-80% flaky filler. The thermoplastic resin may be PET, and the flaky filler may be glass flakes.
U.S. Pat. No. 4,425,457, to Christiansen et al., discloses a composition of 40-95% by weight PET, 4-65% by weight of a reinforcing agent, and 0.1-25% by weight of an alkali metal salt. The reinforcing agent can be glass flake. The alkali metal salt is a nucleating agent. Such compositions can be molded to produce a molded article having a smooth and glossy surface and at the same time sustaining high heat deflection temperature levels.
Japanese Pat. No. Application 58/020636 (as reported in Derwent Abstracts) discloses a plastic container obtained by mixing a thermoplastic resin (e.g. PET) with 0.1-30 wt.% of plate form glass granules. The plastic resin mixture is melted and molded into a container form by extrusion or injection blowing such that part of the thermoplastic container is stretched in at least one direction. The plastic container has excellent thermal shrinkage resistance and mechanical strength.
U.S. Pat. No. 4,693,941, to Ostapchenko discloses a process for thermoforming an article from a sheet of a composition having a crystallinity of 0-5%. The composition consists essentially of 60-77% PET, 3-10% of at least one terpolymer of ethylene, methacrylic acid, and preferably isobutyl acrylate, and 20 to 30 % of a reinforcing filler such as glass flake, with the proviso that the composition does not contain a nucleating agent. The terpolymer of ethylene has 0-100% of its carboxylic groups converted to their salt form, the cation being a divalent metal ion such as zinc. Sheets made from the compositions of this patent are said to have good thermoformability.
With the recent increase in popularity of microwave cooking, it has become common to use trays or dishes made of various plastic materials for both cooking or heating and serving of food. It is important that the plastic material used to make the dishes retains its physical properties at the relatively high temperatures encountered during the cooking process. Even more rigorous high temperature properties are demanded of materials used for dishes which are intended to be suitable for cooking in conventional ovens.
A common material of construction for such trays is crystalline polyethylene terephthalate (CPET). Trays prepared from unreinforced CPET are limited in their use because they are too flexible when removed from a hot oven. Unless such trays are supported on a metal tray, they will warp and sag under the weight of the food. This is because, although PET has a very high melting point (about 250.degree. C.), it loses modulus around 85.degree. C. due to the presence of a glass transition.
Mineral filled CPET compositions, including compositions filled with glass flake, have better stiffness at high temperatures, but tend to suffer from a combination of brittleness and poor color. Glass fiber reinforced compositions tend to be difficult to make and give uneven dimensions. In addition, reinforcing fillers, whether flakes or fibers, are quite expensive, and can make the cost of a reinforced tray prohibitive.
Thermoset polyesters may also be used to make food trays. But the high fabrication costs of thermoset resins make such trays commercially unattractive.
There is therefore a need for a plastic tray that is stiff at elevated temperature, can be easily produced using conventional thermoforming operations, has good color and surface smoothness, and is relatively inexpensive. It has now been found that glass flake reinforcement of at least one outer or near outer layer on each side of a core layer of such plastic trays results in a product which retains excellent stiffness when removed from a hot oven and which is easy to produce at a low cost.